HM Inspectorate of Prisons, Ofsted, Care Quality Commission and General Pharmaceutical Council inspectors found in July to August 2018 that HMP YOI Isis had not acted upon recommendations made two years earlier, and still needed to prioritise support for prisoners aged under 25.
Of the 600 prisoners at the south east London facility, 22 per cent are aged under 21, which since 2016 admits males of all ages. Almost half are aged 21 to 29.
An inspection report following the visit said staff achieved "pockets of good work" with young adult prisoners. However, inspectors found no overall strategy for working with young adults, and the recently implemented violence reduction and individualised education plans policies did not address their needs.
The report states: "Prison procedures did not focus sufficiently on the distinct needs of young adults or consider the impact of punitive measures on them.
"Staff had undertaken a little training on challenging behaviour but more was needed to understand developmental maturity."
Despite improvements in the facility's restriction regime - the average number of prisoners locked up during the working day reduced to 22 per cent of the population compared with 40 per cent in 2016 - inspectors noted that younger prisoners were "disproportionately represented in the statistics relating to force and segregation".
Overall, inspectors found little change at the YOI from its previous inspection. In areas of "safety" and "rehabilitation and release planning", the inspectors gave Isis the same "not sufficiently good" outcome.
Inspectors raised the grade for "respect" to "reasonably good", noting that staff-to-prisoner relationships were mostly good, the prison was cleaner than at the previous visit, and most prisoners were positive about the food.
However, a survey of young people found only 48 per cent of respondents said that most staff treated them with respect, and only 46 per cent could say that they had not experienced any kind of victimisation by staff.
In the area of "purposeful activity'", inspectors reduced the overall outcome to "poor", with the report stating that young people still did not spend enough time in education or training, and those on vocational courses often did not have time to gain accredited qualifications.
Inspectors did praise the YOI's new governor, Emily Thomas, who since her appointment in June 2016 had improved recruitment of staff, and provided "visible leadership" leading to a "more positive culture beginning to emerge".
However, violence levels at the unit remained high, with one in four young people reporting feeling unsafe, particularly younger inmates.
The report states: "The violence reduction strategy lacked sufficient attention to prisoners under 25 who accounted for a disproportionately high number of violent incidents."